It has been well known that hydrogel of a crosslinked polymer can be obtained as an absorbent crosslinked polymer by subjecting a water-soluble ethylenically unsaturated monomer to aqueous solution polymerization in the presence of a slight amount of a crosslinking agent.
The hydrogel of a crosslinked polymer (hereinafter, referred to simply as hydrogel) is a semi-solid gel substance having good elasticity. It is rare to use the hydrogel directly, and in most of the cases, it is divided to fine particles and dried. After the drying step, the hydrogel is pulverized adequately and made into a pulverized product of dry powder, so that it is used as absorbent resin, namely an absorbing agent. Methods of using a thin film dryer, a compartment tray dryer, a drum dryer, a band dryer, etc. are conventionally known as example drying methods adopted in the drying step.
To be more specific, the method using a band dryer is a method, in which hydrogel is placed on a belt conveyer or a screw conveyer equipped with an endless band made of a wire gauze or a porous plate, so that the hydrogel is dried with hot air blown to the same while it is conveyed through the dryer. This method using a band dryer is advantageous in that the hydrogel can be placed continuously without depending on the elasticity or strength of the hydrogel, and further in that a trouble caused when the hydrogel adheres to the dryer happens less frequently.
It is necessary to prevent as much as possible the hydrogel from being delivered to the subsequent pulverizing step before it is dried. The reason is as follows. That is, since incompletely dried hydrogel (hereinafter, referred to as incompletely dried product) is in the form of rubber having good viscosity, the incompletely dried product adheres to a pulverizer and often causes a trouble of stopping the pulverizer during the pulverizing step. Particularly, in case of pulverizing dried hydrogel to particles having a particle size of 1 mm or less, if the dried hydrogel contains the incompletely dried product, the incompletely dried product readily adheres to the pulverizer, and frequently causes a trouble of stopping the pulverizer.
Thus, in order to dry the hydrogel uniformly by the method using a band dryer, it is necessary to place the hydrogel on the band in a layer having a constant thickness. However, in practical applications, it is quite difficult to do so, and avoiding the production of the incompletely dried product is extremely difficult.
A technique disclosed in Japanese Laid-open Patent Application No. 73518/1996 (Japanese Official Gazette, Tokukaihei No. 8-73518, published on Mar. 19, 1996) is known as a method of reducing the production of the incompletely dried product. To be more specific, a pressure of hot air blown to the hydrogel and a pressure at a side opposing the hot-air-blowing side through the hydrogel are measured, so that a thickness of the hydrogel layer on the belt is detected in an on-time basis, according to which the operating conditions of the dryer are controlled. According to this technique, a thickness of the hydrogel layer on the band is measured in an on-time basis economically and effectively, and the operating conditions of the dryer are controlled adequately from time to time. Consequently, the hydrogel can be dried continuously and a dry product of the hydrogel containing a less amount of the incompletely dried product can be manufactured without using any special equipment.
However, the technique disclosed in the above publication can reduce the production of the incompletely dried product, but can not prevent the production of the incompletely dried product completely. Thus, no one has ever achieved a technique to effectively reduce the occurrence of troubles, such as stopping the pulverizer during the pulverizing step.
In other words, the operating conditions of the dryer can be controlled in detail by the method of the above publication, but this method can not prevent the production of the incompletely dried product by eliminating a basic cause because the hydrogel layer can not be maintained at a constant thickness.
To be more specific, in order to place a layer of the hydrogel on the band, aggregates of the hydrogel are often pulverized to fine particles. However, if the particle size of the fine particles of the hydrogel is 10 mm or greater, the incompletely dried product is readily produced, and the production of the same can not be prevented by merely setting the operating conditions of the dryer. Also, in the method using a band dryer, since a material subject to drying (hydrogel) is not stirred or a layer thereof is not turned over during the drying step, some specific portions of the material is hardly blown by hot air. Thus, also in this case, the production of the incompletely dried product can not be prevented by merely setting the operating conditions of the dryer.
Further, since absorbent capacity of the incompletely dried product is lower than that of the adequately dried product of the hydrogel, the physical properties of the resulting absorbent resin are deteriorated if the incompletely dried product is mixed into the dried product of the hydrogel in the pulverizing step.
The production of the incompletely dried product can be prevented completely by drying the hydrogel longer or at higher temperatures so as to eliminate the incompletely dried hydrogel (WO94/09043, published on Apr. 28, 1994). However, this method undesirably extends a drying time or increases operating energy of the dryer. Consequently, manufacturing efficiency of the dried product is deteriorated considerably and the manufacturing costs are increased markedly.
Moreover, according to this method, the hydrogel having been dried earlier is left under high temperature conditions with a slight amount of water until the incompletely dried hydrogel is dried. Thus, the hydrogel dried earlier may be dried exceedingly, thereby deteriorating the physical properties of the resulting absorbent resin.
As has been discussed, in drying the hydrogel, the conventional methods can prevent neither the production of the incompletely dried product completely nor deterioration of the physical properties of the dried product of the hydrogel obtained as the absorbent resin.